(a) Field of the Invention
The present invention relates to a liquid crystal display having a wide viewing angle and using a compensation film.
(b) Description of the Related Art
Liquid crystal displays are typically structured including two substrates provided substantially in parallel with a predetermined gap therebetween, and liquid crystal material is injected between the opposing substrates. Each substrate includes an electrode and an electric field is formed between the substrates by applying a voltage of a different potential to the electrodes. Accordingly, the alignment of liquid crystal molecules of the liquid crystal material changes to control the transmittance of incident light, thereby showing images.
Various types of liquid crystal displays have been developed to improve response times and viewing angle. They include the HAN (hybrid aligned nematic) mode liquid crystal display and the OCB (optically compensated bend) mode liquid crystal display. The OCB mode LCD is arranged symmetrically about an imaginary center plane between the two substrates and parallel to the same. That is, the liquid crystal molecules are aligned substantially parallel to the substrates, then are increasingly slanted until reaching this center plane where the liquid crystal molecules are substantially perpendicular to the two substrates. A wide viewing angle is achieved as a result. To obtain such a bent alignment of the liquid crystal molecules, a horizontal orientation agent that is oriented in the same direction is used and a high voltage is initially applied. Also, the move of liquid crystal molecules in the same orientation when operating, realizes a wide viewing angle as well as fast response times.
However, since liquid crystal material has a birefringence where a refraction index of long axes of the liquid crystal molecules is different from that of short axes of the liquid crystal molecules, a different viewing angle may generate a different refraction index. Accordingly, the ratio of polarization becomes different when the linearly polarized light passes through the liquid crystals. Thus, the amount and color characteristics of the light vary depending on the viewing angle. Hence, color shifting, gray inversion and variations in a contrast ratio happen in the LCD as the viewing angle changes.
An OCB mode LCD is disclosed in U.S. Pat. Nos. 5,410,422 and 5,805,253, in which a compensation film is used to compensate for a phase difference in liquid crystal cells. In these applications, a change in phase of light in liquid crystals is compensated in an opposite direction by the compensation film to solve the viewing angle problem. Here, a uniaxial or biaxial compensation film is used. In U.S. Pat. No. 5,410,422, a retardation value of the uniaxial compensation film is limited to 60-85% of a retardation value of the liquid crystal cells, while a phase difference value of the biaxial compensation film of U.S. Pat. No. 5,805,253 is limited to 0-100 nm.